Background: Copeptin levels reflect vasopressin activity and help classify osmoregulatory disorders. There is limited pediatric experience using copeptin to diagnose and manage diabetes insipidus, syndrome of inappropriate antidiuretic hormone secretion (SIADH), and bi- or tri-phasic postsurgical osmoregulatory disorders. In this report, we describe serial copeptin levels in an infant who developed transient SIADH after neurosurgery. Case Description: A 4-month-old infant with no prior pituitary dysfunction underwent endoscopic fenestration of a large arachnoid cyst (3.5 × 4.7 × 3.8 cm). He developed SIADH on postoperative day 4 with seizures, hyponatremia (sodium 121 mmol/L), and concentrated urine (535 mOsm/kg). His initial copeptin level was inappropriately high in the context of his hyponatremia. Copeptin levels decreased as his SIADH resolved. Serial copeptin levels correlated to the infant’s increased ability to dilute urine. Conclusion: Copeptin levels in this infant are consistent with levels described in adults and older children. Obtaining copeptin levels may improve providers’ ability to quickly diagnose and manage SIADH amongst other heterogeneous causes of hyponatremia. Lastly, trending copeptin levels improved providers’ ability to monitor SIADH progression, and may allow preemptive fluid titration for children with bi- or tri-phasic shifts in osmoregulation after neurological procedures.